Oil burner control



1940- c. T. WALLIS E1 AL 2,201,329

OIL BURNER CONTROL Filed Kay 14, 1938 STACK SWITCH lGNlTlON Patented May21,

PATENT OFFICE OIL BURNER CONTROL Cyril T. Wallis, Rochester, N. Y., andRobert Fowler. Birmingham; Mich., assignors to General MotorsCorporation, Detroit, Mich., a corporation of Delaware Application May14, 1938, Serial No. 207,880

2 Claims.

This invention relates to control apparatus and more specifically toapparatus for controlling fiuid fuel burners for heating purposes.

In utilizing various control systems of the present day for operatingfluid fuel burners for domestic purposes, it is of course necessary tohave in circuit therewith certain safety means and has become rathercommon practice to use what is termed a safety switch for this purpose.

m These safety switches are usually in the form of an electric heatingcoil, the heat generated thereby being applied to a bimetal warp switchwhich upon the application of a certain amount of heat causes a switchto open, said switch requiring manual reset and this assembly has becomeknown as the safety switch.

Since it is actuated by the heat generated by the heating coil and isusually designed to operate its switch after a certain predetermined setinterval, it will be apparent that the amount of current flowing throughthe heating coil will very materially affect the operation with respectto the time required which may be critical.

In the disclosure of our pending application Serial No. 138,688 filedApril 24, 1937, of which this application is a continuation in part, thewarp switch heating coil is placed in series with the burner motor andas such is subject to the various current fluctuations due to motoroperation.

It is therefore an object of this invention to provide regulating meansso that the current flowing through the heating coil will remainsubstantially constant and the time interval will not be alteredregardless of line current variation.

With this and other objects in view, which will become evident as thespecification proceeds, our invention resides in the construction asoutlined in the following specification and claims and illustrated inthe accompanying drawing, in which:

The figure is a schematic wiring diagram of a burner control systemembodying our invention.

Referring now more specifically to the figure, there is shown a suitablepair of incoming lines 88 and 82 for supplying the necessary current towhich a motor 84 is connected which supplies a combustible mixture tothe furnace. An ignition transformer 86 and a main transformer 88 arealso provided and are connected to the source. A room thermostat 9| anda relay 92 complete the conventional control circuit when taken incombination with the safety switch 18 which is composed of a switchmember 38 and a heating coil 54 as well as an auxiliary ignition switch12 which. is also operated bythe heating heater coil 54, motor 84, line98 back to the opposite incoming line 82.

Also connected across the main line is the primary of the transformer 88which is in series with a safety switch 38, through line I02 which isalso connected back to incoming line adjacent the motor switch 81 byline I04.

The motor switch 81 is also connected through line N16 to the ignitionswitch 12 thence through line I08 to the primary of the ignitiontransformer 86 which is then connected back through line to line 80.

Referring now to the secondary or low voltage portion of the controlcircuit we have the secondary of the transformer 88 connected directlyto the thermostat 8| by line H0, the latter being connected by line H2to a stack switch 2| operated by combustion temperatures within theburner and being normally closed when'cold and opening upon a given risein temperature within the furnace. This latter switch is then connectedthrough line H4 to relay 82 and thence back to the secondary of thetransformer 88 through a line Hi. There is also connected across fromline I 12 to line I I4 a shunt line I I8 having therein a holding switchI20 operated by the relay 92.

It is therefore obvious that with the switches 2i, l2 and 38 normallyclosed during inoperative periods, when the room thermostat 8| closesits contacts to call for heat the relay 92 will be energized through anobvious circuit and close switches 81 and I20, the first completing acircuit through the motor and also through the ignition coil and thesecond closing a holding circuit for the relay itself so that when thetemperature within the burner increases and stack switch 2| opens therelay will still remain energized if the room thermostat is closed.

After a certain period of heating of the coil 54 through which the motorcurrent flows, the switch 12 is designed to open thereby cutting offthe.

ignition but switch 88 is prevented from opening after a further periodby a certain interlock (not shown herein) operated in conjunction withthe stack switch. It is therefore obvious that normally the total motorcurrent in this construction would pass through the heating coil 54 forthe warp member and therefore the heating effect thereof is subject tothe fluctuations in the amount of current drawn therethrough by themotor. As an example of this fluctuation, one of the present commercialtype of burner motors has been found to have an input variation of from1'75 to 190 watts when the motor is new, or approximately 8%. Of courseif the motor is old and other losses increase, the variation may behigher. It has also been noted that the line voltage on power supplysystems may vary 115%, depending upon the load and feeder size. There isthus a possible variation of 38% in current input to the motor. Thenormal motor current is 2.5 amperes. A plus or minus 19% variation wouldmean a variation of from 2.03 to 2.97 amperes. As the current throughthe motor circuit varies, the heat produced by coil 54 will also varyand therefore the time of operation of the bimetal arm will vary. As anexample of this, one of the resistors used had a value of 1.6 ohms andtherefore with the above current variation the variation in dissipationin watts in the resistor would be 6.6 to 14.1 watts. This might causethe ignition to be deenergized too soon or might cause the warp switch38 to open before the stack interlock has had suflicient opportunity tomechanically prevent this.

In order therefore to overcome this and to provide for a definite settime interval under any current variation a reactor coil 55 is placed inparallel with the heating coil 54 to act as a regulator therefor. Thisreactor coil since it is placed directly in parallel with the resistanceelement 54 is'subject to the same voltage and the reactor thereforefunctions as an automatic voltage regulator. The value of the reactor,as an example,

has a voltage drop across it of 4 volts at line voltage. The core 5'! ofthe reactor is so designed that when the motor current flowing in thecircuit is normal the core is nearly saturated and hence the impedanceis substantially constant.

Therefore since the voltage drop across the reactor is a function of theimpedance in circuit, the proper amount and grade of iron for the coremay be selected to insure a substantially constant voltage for aspecified current variation. Therefore since the voltage across thereactor coil is maintained substantially constant, that across theresistor coil is the same and the heat produced by the heating coil andthe time of operation of the bimetal arm will always remain the same.

We claim:

1. In a control mechanism for fluid fuel burners, electrically operatedmeans for discharging a combustible mixture into a zone of combustion, athermal time switch controlling the means and an electric heating coilsupplying heat thereto and in series with said first-named means, andmeans regulating the current through the heating means to maintain thesame substantially constant.

2. In a control mechanism for fluid fuel burners, an electric motor fordischarging fuel, a thermal time switch controlling motor operation, aheating coil for the time switch in series with the motor and a reactorcoil in a parallel circuit to the heating coil for maintaining thecurrent through the heating coil substantially constant irrespective offluctuations in the motor current.

CYRIL T. WALLIS. ROBmT E. FOWLER.

